Equity Incentive Plans - Number and Weighted Average Grant-Date Fair Value of Unvested Stock Options (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Outstanding Beginning Balance, Number of Options	897,421	631,997	455,543
Granted options during the period, Number	70,865	827,406	802,193
Vested options during the period, Number	(455,301)	(446,570)	(532,682)
Forfeited options during the period, Number	(19,031)	(85,934)	(92,959)
Exercised options during the period, Number	(18,029)	(29,478)	(98)
Outstanding Ending Balance, Number of Options	475,925	897,421	631,997
Unvested options at the beginning of the period, Weighted Average Grant-Date Fair Value	$ 1.72	$ 2.40	$ 4.18
Granted options during the period, Weighted Average Grant-Date Fair Value	5.02	1.54	1.67
Vested options during the period, Weighted Average Grant-Date Fair Value	1.74	2.26	2.56
Forfeited options during the period, Weighted Average Grant-Date Fair Value	1.77	1.88	4.01
Exercised options during the period, Weighted Average Grant-Date Fair Value	1.59	1.24	3.08
Unvested options at the end of the period, Weighted Average Grant-Date Fair Value	$ 2.19	$ 1.72	$ 2.40